98 Meurer Proc. ASA 9 (1) 1991 Mass Loss in Dwarf Galaxies Gerhardt R. Meurer, Anglo-Australian Observatory, PO Box 296 Epping, NSW 2121 Abstract '. Four examples of dwarf galaxies apparently un- dergoing mass loss are discussed. All have an Ha morphology suggesting a minor axisflow.This is supported by velocity field measurements in three cases. All also have at least one compact central object, probably a young star cluster, apparently pow- ering the flow. Some implications of these results are briefly discussed. 1. Introduction Dwarf galaxies can be put into two broad categories: gas-rich and star-forming, and gas-poor and 'inactive'. The former group consists of dwarf irregular (dl) and blue compact dwarf (BCD) galaxies, while the latter group consists of dwarf spheroidal (dSph) dwarf elliptical (dE) and compact elliptical (cE) galax- ies. Except for the cEs most dwarf galaxies have an exponen- tial radial surface brightness distribution, at least in their outer regions (e.g., Bothun et al. 1986). Therefore, the main require- ment for evolution from the star-forming to the inactive group, is a depletion of the gas content. It has long been realised that the interstellar medium in dwarf galaxies can be accelerated to escape velocity by the action of supemovae (e.g., Larson 1974). However, 'galactic winds' in dwarf galaxies have often not been recognised as such. Here evidence is discussed for mass loss in four dwarf galaxies. Basic data on these galaxies are given in Table 1. Ha images indicative of a wind can be found in the references quoted below. Table 1. Mass loss dwarf galaxies: basic data Galaxy NGC 1705 NGC 1569 IZwl8 NGC 5253 Type amorphous/BCD amorphous BCD amorphous/BCD Distance (Mpc) 4.7 2.9 10.0 3.7 MB T -15.7 -16.2 -14.4° -16.8 All data from references quoted in the text except: ° Searle and Sargent (1972) 2. Examples (a) NGC 1705 This galaxy has been studied in detail by Meurer (1989; see also Meurer et al. 1989). In red continuum light NGC 1705 shows a dE-like morphology with a slightly off-centre 'nu- cleus'. This object (MB = —13.9) is actually a very young (~ 13 Myr old) star cluster with a mass (~ 1.5 x 10 6 MQ) like the most massive galactic globular clusters. The Ha morphol- ogy is that of a loosely collimated bipolar flow centred on the nucleus and aligned roughly along the continuum minor axis. Split emission lines (At; « 100 km s -1 ) are seen over much of the galaxy including three components at the nucleus. Only the most blue-shifted of these is seen as an absorption component of Al It A 1671A against the nucleus (York et al. 1990) indicating the flow is expulsive. The expansion time-scale of the flow is within a factor of two of the age of the nucleus for all likely geometries. The evidence thus clearly points to the bipolar flow being powered by the nucleus. Calculations show that the mechanical energy release of the massive stars in the nucleus, in the form of stellar winds and supernovae, can provide the observed kinetic energy of the flow. For realistic mass to light ratios some gas has reached escape velocity with an estimated mass loss rate of M > 1 MQ yr -1 . (b) NGC 1569 Hodge (1974) presents narrow band continuum and Ha pho- tographs of this amorphous galaxy. Short exposures show two compact central clusters {MB = —13.3 and —12.5; discussed in detail by Arp and Sandage 1985) which have intrinsic B — V colours like the nucleus of NGC 1705, indicating ages of around 10 Myr. The Ha morphology isfilamentary,with most of the fil- aments radiating out the continuum minor axis. De Vaucouleurs et al. (1974) suggest a bipolarflowmodel for their observed Ha velocity field. The expansion time-scale of the flow is about 10 Myr, i.e., roughly the age of the central clusters. (c)lZwl8 Dufour and Hester (1990) have published continuum and Ha images of this well studied BCD. The continuum light distribu- tion is along a line. Four unconnected blobs are aligned along this axis, as is the major axis of the resolved blobs. The Ha distribution is aligned perpendicular to the continuum axis and appears to be made up of giant loops. The centre of the Ha distribution is a bright compact object in the main blob, which may be barely resolved. The Hi velocity field of Viallefond et al. (1987) shows a significant velocity gradient along minor axis spikes (which appear to constrain partially the Ha loops), suggesting a minor axis flow. (d) NGC 5253 Good images of NGC 5253 are presented by Caldwell and Phillips (1989). In its centre there are five bright blue knots with unresolved cores. They have absolute magnitudes and UBV colours like the ionizing cluster of 30 Dor, NGC 2070 (MB = —11.1). In Ha filaments and bubbles are seen, predom- inantly radiating out the minor axis. 3. Common Factors All examples have central compact sources, probably young star clusters, and minor axis Ha filaments. At faint light levels well defined Ha bubbles can be seen with kpc-scale diameters. This is an expulsive morphology as confirmed by the absorption line results in NGC 1705. The morphology and coincidence of the cluster ages and Ha expansion time-scale for two of the galaxies is strong evidence that a young star cluster can power the minor axis flow. Three of the examples are classified as 'amorphous' (Sandage and Brucato 1979). Nearby (D < 5 Mpc) BCD galaxies will not appear compact but have an appreciable angular extent. These galaxies are likely to be classified as amorphous. 4. Implications (a) Opening a closed box Many BCDs such as I Zw 18 have low metallicity and yet a high degree of formation into stars. This is one of the chief arguments for BCDs being young or undergoing a burst of short duration: if they formed the stars over a Hubble time they would be more chemically evolved (e.g., Searle and Sargent 1972). However, this assumes a 'closed box', that is all gas is retained and well https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1323358000025042 Downloaded from https://www.cambridge.org/core. IP address: 172.81.112.189, on 21 May 2019 at 09:02:34, subject to the Cambridge Core terms of use, available at